Field of the Invention
The invention is in the field of analog television systems such as the NTSC and MAC video systems and specifically in techniques and implementing apparatus for establishing horizontal and vertical synchronization and other reference levels for reproducing video information at the receiver.
In conventional television systems video information is transmitted in frames, each frame consisting of a plurality of horizontal lines. In a typical known system, each frame consists of 525 horizontal synch pulses, and the frame intervals to vertical synch pulses. FIG. 1A shows a typical example of a color TV signal including the vertical and horizontal sync pulses, video information, equalizing pulses and color burst. The type of signal shown is conventional and would appear in a normal TV transmission system. The particular format of the waveform shown is that which would occur for an interlaced scanning system in which each frame is 525 lines long. As it is illustrated in the diagram, the prior frame terminates at point X on the graph after the 507th horizontal line of video information preceded by horizontal synch pulse H 507, and the new frame begins at the same point. The frame begins with six equalizing pulses followed by six vertical synch pulses followed by six more equalizing pulses. The vertical synch pulses and the equalizing pulses are separated by a distance H/2, where H is the horizontal line time. Typically the equalizing pulses will be 2.4 microseconds in width and the vertical synch pulses will be 27 microseconds in width. The group of 12 equalizing pulses and six vertical synch pulses which follows the beginning of the frame will be referred to hereinafter as the Field I or odd field synch group. The designation is hereinafter used only for the purpose of distinguishing between the two groups of equalizing and vertical synch pulses, the first group preceding the first field of the frame and the second group preceding the second or even field of frame.
Following the last equalizing pulse of the Field I synch group are a plurality of horizontal synch pulses (254 in the particular example described) which are separated by a distance H. It should also be noted that the first horizontal synch pulse following the last equalizing pulse is separated therefrom by distance H/2. The color burst information, if there is color transmission, and the video information for the particular line, follows the horizontal synch pulse for the line.
The last horizontal synch pulse within the first field is followed by the Field II or even field synch group which comprises six equalizing pulses followed by six vertical pulses followed by six more equalizing pulses. The first equalizing pulse within the Field II synch group is separated from the beginning of the last horizontal synch pulse 254 within the first field by the distance H/2. Following the last equalizing pulse of the Field II synch group are the remaining horizontal synch pulses and associated video information. Since the diagram represents the television transmission signal used in an interlaced scanning TV system, the first horizontal synch pulse follows the Field I synch group by H/2 whereas the first horizontal synch pulse in the second field follows the Field II synch group by distance H. The converse relation, as can be seen in the diagram, is true for the last horizontal pulse in each field and the Field I and II synch groups.
Since the frame time is 525 H. and since each field synch group occupies a space of 9H, there will be 507 horizontal synch pulses per frame. It will be noted from the diagram that the first few horizontal synch pulses following each field synch group are inactive, i.e., no video associated therewith. There will be about 17 inactive synch pulses per frame. This is conventional in TV transmission.
A portion of the total waveform diagram representing the horizontal synch pulses and the associated video is illustrated in FIG. 1B. As shown in that figure, each horizontal line of approximately 63 microseconds includes a horizontal blanking signal during a horizontal blanking interval of approximately 10 microseconds. This blanking signal includes a 1.27 microsecond front porch, followed by a 4.75 microsecond horizontal blanking pulse, followed by a color burst frequency (if color transmission is involved). The line video information follows the color burst. The color burst, that is, the color subcarrier used for clock phase synchronization may typically consist of several cycles of a 3.58 MHz sinusoidal waveform and is necessary to very precisely maintain the phase relationship of the transmit and receive side clock signals, thereby preventing color distortion.
It is well known that analog TV systems require DC restoration and automatic gain control at the receiver. Typically, the DC restoration procedure involves the transmission of a reference level usually in the horizontal blanking interval. At the receiver, the received level is clamped in a DC level clamping circuit to the original level represented by the reference level transmitted during the horizontal blanking interval. The typical gain control technique involves the transmission of a second reference level. Assuming that the DC level has been properly restored, the received signals are then forced to the proper levels through gain adjustment after measuring the difference between the received second reference level and the actual second reference level generated at the receiver.
The disadvantage of the conventional horizontal synchronization technique is the requirement for a level which is not used in the video signal. This translates to additional power requirements of approximately 3 dB in either AM or FM analog systems. The disadvantage of the aforementioned DC restoration and automatic gain control, AGC, techniques is their requirements for specific reference levels to be transmitted in the horizontal blanking interval. The time intervals used to transmit these reference levels are then unavailable for other purposes. Indeed, in a conventional system, about 16% of a complete horizontal line period is spent for retrace, synchronization and reference level determination purposes.
In U.S. Pat. No. 3,666,888 which issued on May 30, 1972 to Sekimoto and which is assigned to the assignee of the present invention, there is disclosed a digital TV system and specifically a PCM-TV transmission system in which the horizontal synchronizing signal is not transmitted. Rather a unique digital word is transmitted for every horizontal line in place of the horizontal synchronizing signal to convey the horizontal synchronization timing information. The Sekimoto unique word is 20 or 30 bits long and can convey the horizontal synchronization timing information in a time interval significantly shorter than the horizontal blanking interval available for other purposes, such as transmitting audio and data channels. Use of a horizontal unique digital word for horizontal line synchronization information is, however, not known in the analog TV art where the video information as well as the vertical and horizontal synchronization timing information are transmitted as modulated analog signals.
While the transmission of a horizontal unique digital word for horizontal synchronizing timing information would free up portions of the horizontal blanking interval for digital audio and data channels, there still remains the problem that in analog TV systems substantial portions of the horizontal blanking interval must remain available for the color burst clocking signal, and for the DC restoration and AGC reference levels and therefore, substantial portions of the horizontal blanking interval remain unavailable for audio and data channels.